(1) Field of the Invention
The present invention relates to a method and to a rotary wing aircraft optimized to minimize the consequences for the occupants of said aircraft during a running off-specification emergency landing.
(2) Description of Related Art
Such an aircraft has a rotary wing provided with at least one rotor providing at least part of the lift of the aircraft, and possibly also providing the aircraft with propulsion when the aircraft is a helicopter.
The aircraft can then land vertically while its speed of advance is zero, or it can perform a so-called “running” landing in which it lands with a positive speed of advance.
A rotary wing rotor has a plurality of blades. The person skilled in the art uses the term “advancing” blade to designate each blade that is moving forwards relative to the movement in translation of the aircraft, and uses the term “retreating” blade to designate each blade that is moving rearwards relative to the movement in translation of the aircraft. Thus, depending on its azimuth angle, each blade is in an advancing blade position over half a revolution and then in a retreating blade position over the following half revolution.
For convenience and throughout this text, it is assumed that the left of an aircraft lies on the left of a person sitting on a longitudinal anteroposterior plane of symmetry of the aircraft with his or her back towards the rear of the aircraft so as to be looking towards the front of the aircraft. Consequently, it is assumed that the right of the aircraft is situated on the right of said person.
Under such circumstances, when the rotary wing aircraft is moving forwards and the rotor blades are turning clockwise when seen from above, the blades on the left of said person are blades in an advancing position, while the blades on the right of that person are blades in a retreating position. Each blade alternates between the status of a blade in the advancing position and the status of a blade in the retreating position. Naturally, this effect would be inverted if the rotor were turning counterclockwise.
Conventionally, a rotary wing rotor is driven in rotation by a main power transmission gearbox, known under the acronym MGB, the power transmission gearbox itself being driven by a power plant. The power transmission gearbox is fastened to the structure of the aircraft by fastener means, such as sloping fastener bars.
Under such conditions, one of the events to be feared during an accident involving a rotary wing aircraft under non-regulation conditions, is the possibility of at least one of the blades of the rotary wing making contact with the ground.
The blades of the rotary wing run the risk of coming into contact with the ground under accident conditions. This sometimes leads to breakage of the members or of the materials that ensure the blades present stiffness in flapping and in the lead-lag direction.
Consequently, a blade coming into contact with the ground runs the risk of subsequently performing large movements that might lead to interference with the aircraft cabin or with some other structural element of the aircraft, such as its tail boom.
More precisely, it can be understood that the blades in the advancing position run the risk of impacting the cockpit, whereas the blades in the retreating position run the risk of colliding with the tail boom, for example.
Although a collision between a blade and the tail boom will lead a priori to damage that is purely structural, it can be understood that such contact between a blade and the aircraft cabin is nevertheless likely to injure the occupants of the cockpit.
Thus, under emergency landing conditions going beyond the conditions specified by certification regulations, an accident can lead to a blade colliding with the aircraft cabin.
Pilots therefore naturally tend to ensure that the aircraft rolls towards the side where the blades are in the retreating position in order to avoid such a collision.
During a crash taking place at high speed, e.g. at a vertical speed greater than six meters per second, a collision between the blades and the aircraft cabin can be caused by heavy masses being projected forwards, and in particular by the MGB that drives the rotary wing being projected forwards.
In contrast, during a crash that takes place at a slower speed, e.g. a vertical speed lying in the range three to six meters per second, collision of the blades with the aircraft cabin may be caused by breakage of landing gear.
More precisely, the landing gear of an aircraft usually comprises at least two means for making contact with the ground, which means are connected to the fuselage of the aircraft by fastener devices.
In a first embodiment, the landing gear comprises at least two contact means, each having at least one wheel. The wheels of the contact means are connected to the fuselage by fastener elements sometimes referred to as undercarriage “legs”.
An undercarriage leg may include an arm that optionally co-operates with a shock absorber or indeed a retraction actuator, for example.
In a second embodiment, the landing gear comprises at least two contact means, each contact means having a respective skid.
Each skid is connected to the fuselage by two fastener devices, e.g. each comprising a fastener zone of the skid, a branch of a cross-member, and a sleeve connecting said branch to said fastener zone. In addition, the branch is connected to the fuselage by means of a fastener member.
By way of example, two-skid landing gear has a front cross-member and a rear cross-member, each cross-member having two branches for connection to the two skids.
Under such circumstances, the landing gear may have four fuselage fastener members, i.e. one fastener member per branch.
In an alternative, the landing gear has three fastener members, two fastener members being fastened to respective ones of the two branches of one of the cross-members, and one fastener member being fastened substantially to the junction between the two branches of the other cross-member.
Under such circumstances, and independently of the nature of the landing gear, each fastener device is dimensioned to present strength in its failure mode that enables it to withstand the emergency landing conditions specified by certification regulations, possibly as increased by a safety margin or safety coefficient. This strength is referred to for convenience as the “regulation strength”.
It should also be observed that, for convenience in the present certification, the term “regulation emergency landing conditions” is used to designate the emergency landing conditions as defined by certification regulations and also those conditions as made more severe by the manufacturer for safety reasons, where applicable. Under such circumstances, the emergency landing conditions defined by certification regulations, whether or not made more severe by the manufacturer, are referred to for convenience as “regulation emergency landing conditions”.
In contrast, emergency landing conditions not covered by said certification regulations are referred to as “off-specification emergency landing conditions”.
Landing gear is then dimensioned to withstand regulation emergency landing conditions. Nevertheless, an aircraft may be confronted with emergency landing conditions that are off specification. Under such off-specification emergency landing conditions, a fastener device of the landing gear may accidentally break, which in turn may lead to a blade of the rotary wing coming into contact with the ground.
It can also be understood that the aircraft may need to land on an inhospitable surface with obstacles of a kind that will lead to off-specification emergency landing conditions, such as obstacles constituted by rocks striking the landing gear during a running landing, for example.
The technological background includes in particular the following documents: FR 2 537 542, JP 2009/073209, FR 2 895 368, FR 1 549 884, certificate of addition FR 2 010 302, and document FR 2 554 210.
Document FR 2 537 542 presents skid landing gear fitted with components including an energy-absorber device that absorbs energy by plastic deformation and/or by force limiting.
Document JP 2009/073209 discloses skid landing gear having an energy-absorber device with elastic members.
Document FR 2 895 368 discloses a rotorcraft having landing gear. The landing gear is connected to the fuselage by connection means that present angular stiffness that varies with the landing gear as shortening a result of making contact with the ground.
Document FR 1 549 884 and certificate of addition FR 2 010 302 disclose energy-absorber devices that absorb energy by plastic deformation and by force limiting, those devices including rolling elements such as balls or wheels arranged between two telescopic members. Thus, in the event of an impact causing the two members to move telescopically, the rolling devices lead to progressive plastic deformation of at least one of the two telescopic members.
Document FR 2 554 210 discloses an energy-absorber device that absorbs energy by elastic deformation of landing gear.
Also known is document EP 0 072 323.